The present invention relates to diphosphines, a process for their preparation, metal catalysts derived from them and the use of such catalysts.
There has been much interest in the asymmetric hydrogenation of alkenes in recent years using, in particular, rhodium catalysts derived from P-chiral diphosphines. There is a need to improve such processes so as to enhance the enantio-selectivity.
It is commonly believed that C2 symmetric diphosphines along with diols and diamines are endowed with superior properties as ligands in catalysis and this is, of course, augmented by their ease of synthesis. According, to the present invention, we have surprisingly found that excellent results can be obtained by a novel class of unsymmetrical diphosphines.
Accordingly the present invention provides a non-symmetrical diphosphine of the formula
R1R2Pxe2x80x94(Z)xe2x80x94PR3R4
wherein Z represents a chain of 2 to 4 carbon atoms which may be substituted, which chain may be saturated or unsaturated, eg. ethylenically unsaturated, R1, R2, R3 and R4, which may be the same or different, are aliphatic, aromatic or heteroaromatic groups attached to the phosphorus by carbon, nitrogen, oxygen or sulphur such that each phosphorus atom and its substituents independently form a single enantiomer. It will be appreciated that, in general, there is a single stereochemical configuration around each phosphorus atom. Thus one or both phosphorus atoms may form a chiral centre. Suitable substituents of Z are hydrogen or aliphatic, aromatic or heteroaromatic groups.
Preferably the diphosphines are 1,2-ethanes ie. the carbon chain is xe2x80x94(CH2)2xe2x80x94. Other typical Z groups include those having the chain structure xe2x80x94Cxe2x80x94Cxe2x95x90Cxe2x80x94C and xe2x80x94Cxe2x80x94Cxe2x95x90Cxe2x80x94.
Generally, the substituents R1, R2, R3 and R4 will be connected to the phosphorus atoms by carbon atoms. In a preferred embodiment, R1 and R2 and/or R3 and R4 are linked together to form the substituted or unsubstituted 3,4,5,6 or 7 membered phosphorus heterocycle and preferably a phospholane ie. a five membered ring. This ring desirably has the formula 
wherein R5 and R6, which may be the same or different, are hydrogen, hydroxy or C1 to C4 alkoxy and R9 and R10, which may be the same or different, are hydrogen or C1 to C4 alkyl.
It is also preferred that R1, R2, R3 and/or R4 are substituted or unsubstituted phenyl, the substituents preferably being hydroxy or C1 to C4 alkoxy groups.
The alkyl and alkoxy groups are typically methyl and methoxy, respectively.
It will be appreciated that although the diphosphines are non-symmetrical R1, R2, R3 and R4 may all be the same provided that the stereo orientation of R1 and R2 on the one hand, is different from that of R3 and R4. The values of R1, R2, R3 and R4 must be such that each phosphorus atom and its substituents independently form a single enantiomer.
Preferred diphosphines of the present invention have the formula 
wherein R5 and R6, which may be the same or different, are hydrogen, hydroxy or C1 to C4 alkoxy.
In accordance with another aspect of the present invention these diphosphines can be obtained in optically pure form rather than as a mixture of isomers.
It is usually convenient if at least one of the phosphorus atoms is ligated to a borane. This enhances the storage stability of the phosphine. It will be appreciated that it is a simple matter to de-boronate when it is desired to generate the ligand. Catalysts can be obtained from the diphosphine with a, generally low valent, metal such as rhodium, iridium, ruthenium, palladium or platinum. The ligand can be reacted in known manner to generate the catalyst. For example a rhodium catalyst can be obtained by reaction of the ligand with (COD)2RhBF4. By xe2x80x9cCODxe2x80x9d, as used herein, is meant cyclooctadiene. The preparation of the catalysts from the ligand can be obtained in known manner as one of skill in the art will appreciate.
The catalysts of the present invention are generally neutral or cationic complexes. Typical counterions which can be present if they are cationic include halide, for example fluoride or chloride, tetrafluoroborate, hexafluorophosphonate, hexafluoroantimonate, or sulphonate of formula R7SO3 where R7 is an aliphatic or aromatic group, or boronate of the formula (R8)4B wherein the R8 groups which may be the same or different are aromatic groups. The aromatic groups are typically phenyl groups which are optionally substituted. When R7 is aliphatic it is typically an alkyl group, for example of 1 to 4 carbon atoms such as methyl.
The non-symmetrical diphosphines of the present invention are generally prepared by a Michael-type addition reaction of a nucleophilic phosphorus-containing reactant with an unsaturated, preferably an ethylenically unsaturated, phosphorus-containing reactant or a cyclopropyl phosphorus-containing reactant.
The nucleophilic phosphorus-containing reactant may be any compound of the formula
R11R12PH
wherein R11 and R12, which may be the same or different, are aliphatic, aromatic or heteroaromatic groups attached to the phosphorus bycarbon, nitrogen, oxygen or sulfur. The nucleophilic phosphorus-containing reactant may also be an organometallic derivative of the formula
R11R12PM
which may be ionic or covalent, and in which R11 and R12 are as defined above and M is a suitable metal.
Preferably the nucleophilic phosphorus-containing reactant is an enantiomerically pure phosphine and most preferably it is an enantiomerically pure phosphine borane such as ortho-anisylphenylphosphine borane.
A phosphorus atom with electron-withdrawing substituents, attached to a double bond results in the alkene being responsive to nucleophiles. The unsaturated phosphorus-containing reactants suitable for use in the present invention may be oxidised phosphorus-bonded alkenes, for example diethyl vinylphosphonate, which may later be reduced to provide a primary phosphine. The alkene is preferably ethene or 1,3-butadiene.
The diphosphines of the present invention are typically prepared via a diphosphine intermediate comprising a primary phosphine and tertiary phosphine.
The primary phosphine may be elaborated by reaction with a doubly electrophilic carbon moiety which can provide a source of chirality giving an enantiomerically pure product. It may be converted into a phosphorus heterocycle by reaction with a diol activated by conversion of the hydroxyl groups into leaving groups. The diol may be activated by, for example, conversion into a halogen derivative, sulphate, sulfonate or phosphate. Diols suitable for use in the present invention include C2 to C6 diols. The diols may be unsaturated or saturated and they may optionally be substituted by oxygen, nitrogen, sulfur, aliphatic, aromatic or heteroaromatic groups.
It will be appreciated that other substituents may be attached to the primary phosphine in an analogous manner.
In the process of the present invention it is advantageous to convert one or both of the phosphorus atoms into, for example, oxide or sulfide derivatives, preferably borane derivatives, which may later be converted back into the desired phosphine or diphosphine.